Humans, monkeys and mice can all catch the urge to itch from seeing others scratch.
Photograph: Rengim Mutevellioglu/Getty Images

Humans and monkeys are not alone in finding they can catch an itch. Researchers have discovered that mice also start scratching when they see others at it – and worked out why.

Contagious itching has long intrigued scientists, with various suggestions cropping up to explain the phenomenon, among them the possibility that it might help social animals curb the spread of parasites. Previous research has also suggested that it is more prevalent among highly neurotic people.

But little was known about the mechanism behind the behaviour.

“Scratching is an unconscious behaviour evoked by an uncontrollable urge – not a decision – therefore this is what we call innate behaviour, a basic instinct,” said Zhou-Feng Chen of the Washington University school of medicine, a co-author of the research published in the journal Science. But the question, he says, remained: “How is it programmed in your neurocircuitry in your brain?”

In an attempt to unpick the answer, Chen and colleagues from the university’s Center for the Study of Itch, began by exploring whether mice, like monkeys and humans, could catch the urge to scratch.

To do so, rodents were either put in a cage adjacent to mice with chronic itch, or were given neighbours without the condition. Within five seconds of catching sight of a mouse with chronic itch scratching, the neighbouring mice significantly increased their own scratching behaviour. By contrast those next door to mice without the condition showed no signs of increased scratching.

The results were similar when the mice were shown to their peers on a video screen, rather than in the flesh – a surprise, given that mice are known to have poor vision. The results, say the team, rule out sound and smell as being behind contagious itching, instead suggesting it is rooted in seeing others scratch.

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The findings also suggest that the behaviour is not linked to empathy, says Chen, noting that mice only tend to show empathy towards mice they are familiar with.

Delving deeper, the team sought to unpick the mechanism behind the copycat scratching. The team found that in mice that had watched others scratching, brain activity increased in particular regions, including the suprachiasmatic nucleus – a region that controls the so-called “body clock” and is linked to jet lag. “No one suspected [this region] to also have an important function in vision,” said Chen.

Further analysis revealed that watching others scratch appeared to result in a depletion in levels of a neuropeptide called gastrin-releasing peptide in the suprachiasmatic nucleus. “The [neuropeptide] has been used as a trigger of scratching behaviour,” said Chen. Previous work by the team has shown the neuropeptide transmits signals relating to itching between the skin and spinal cord.

The researchers also showed that when mice could not produce or detect the neuropeptide, they did not catch itching. When the neuropeptide was injected into them, they began scratching without seeing others do the same.

“This just another [piece of] evidence to tell you [itching] is hard-wired in the brain, because it is like a chain reaction. It is an automatic event,” said Chen.

While it is unclear if the same mechanism underpins contagious itching in humans, Chen believes it is likely a similar pathway is at play. He believes similar mechanisms might also be behind other contagious behaviours, such as yawning.

Anil Seth, professor of cognitive and computational neuroscience at the University of Sussex who was not involved in the research, described the study as a technical tour de force, pointing out that it suggests “very deep and evolutionary ancient mechanisms might be involved” in contagious itching.

But it has been difficult to establish a link between empathy and contagious itching in humans, and Seth says it is not clear if the newly revealed mechanism could alone explain the phenomenon.

“In humans, contagious itching is accompanied by subjective feelings of itchiness and involves a whole range of brain areas associated with processing of touch and emotion,” he said.